The present invention relates to valves, and more particularly to electric vent valves used in lubrication systems.
Certain lubrication systems require a vent valve to release lubricant pressure upon completion of a lubrication cycle. Such a system may be a “single line” lubrication system that includes one or more lubricant dispensers (e.g., injectors) to dispense lubricant to one or more devices (e.g., bearings), a supply of lubricant, a supply line fluidly connecting the supply with the dispenser, and a pump to pressurize lubricant in the supply line to flow from the supply to the dispenser(s). Typically, a controller is provided to initiate pump operation when lubrication is required and to shut off the pump when a desired maximum lubricant pressure in the dispenser(s) is reached, as determined typically with a pressure switch.
Further, the vent valve is provided to “bleed off” pressure in the supply line once the desired maximum lubricant pressure is achieved, and may be hydraulically, pneumatically or electrically driven. Previously known electric vent valves typically include a spool driven by a solenoid that moves the spool between open and closed positions. These valves are normally open and are driven to close when a lubrication cycle is initiated. Due to the relatively high pressures reached in the lubricant supply line, particularly when the lubricant is grease as opposed to lower viscosity oils, the solenoid must maintain the spool in the closed position against a substantial force acting on the spool that tends to push the spool toward the open position. The solenoid force required to maintain the spool at the closed position against such high pressure is generally difficult to achieve with a typical low voltage power supply (e.g., 24 volts). Furthermore, due to the relatively short stroke of a typical solenoid, the valve orifices must be made relatively small, which leads to rapid erosion of metal parts of the valve particularly when handling high pressure grease.
With prior art solenoid type valves, the relatively small size of the orifices restricts the flow of grease during the bleed off phase of the lubrication cycle. In cold temperature climates, particularly with a relatively “stiff” grease, such small orifices can prevent the grease from flowing through the valve back to the grease supply; thus the bleed off phase is not achieved. Unless the bleed off phase is complete, the lubrication system can not function. Typically, the pressures in the lubrication system reach up to 3,500 psi up to a maximum of 5,000 psi to operate the lubrication valves (injectors) and then must bleed off to less than 400 psi. The small orifices also subject the valve to clogging if even small amounts of contaminants are present in the grease.